Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

moderate aspect ratios. This suggest that further gains may be possible byusing even longer, wirelike rods [128,129] or by using other nonsphericalnanostructures which can now be reproducibly synthesized [130]. Indeed, theauthors found that devices with moderately sized (8 13-nm) nanorods couldconvert five times as much optical radiation into electrical power as devicesmade from smaller (4 7-nm) nanorods.In some cases, polymer–nanocrystal blends have also been shown toundergo phase separation in the direction perpendicular to the plane of thefilm [118]. Such vertically graded structures can help optimize charge generationand charge collection efficiency, as has been shown in solution-processedmolecular films [125]. Furthermore, adjusting the surface energyinteractions of nanorods and a host polymer can be used to control filmmorphology in a rational manner [131].Finally, although we have largely neglected quantum-confinementeffects in our discussion of nanocrystal photovoltaic devices, one can envisionscenarios where they could be used to tune the response of a detector to aspecific wavelength or to optimize energy level offsets in order to maximizeopen-circuit voltages. Even though these effects have not yet been fullyexploited, the study of quantum-confinement effects has led to the ability tocontrol the size, shape, and surface of these chemically synthesized quantumdots, properties which offer compelling advantages to the development ofsolution-processable photovoltaic devices.VII.CONCLUSIONSNanocrystals provide an interesting system in which to study the physics ofcharge transport at the nanoscopic level. Both electronic tunneling and structuralreorganization play important roles in determining the rate of chargetransport, and we have shown earlier how the charge transport is expectedto change as the nanocrystal size and spacing are varied. Experimental measurementsof charge transport in nanocrystalline films have allowed mobilitiesto be measured and have identified the importance of disorder andtrapping in determining the macroscopic charge transport properties.Nanocrystals also allow the study of photoinduced electron transferfrom organic molecules and polymers to semiconductors, because a largeinterfacial area is present at the nanocrystal surface where charge transfer cantake place. We have shown that CdSe nanocrystals act as good electronacceptors from many conjugated polymers, providing rapid electron transferfrom the photoexcited polymer to the nanocrystal, followed by slow recombinationof the charge-separated state. This charge-separation process can beexploited as the first step in the operation of a photovoltaic device based onCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.